This invention relates to microwave filters and in particular to a microwave cavity filter having a thermal compensation arrangement.
With increasing demands on the radio frequency spectrum, microwave cavity filters are required to be highly selective. In order to ensure high selectivity, the filter""s electrical characteristics must be maintained during temperature fluctuations.
If a microwave cavity filter having one or more cavities is made from a material having a high coefficient of thermal expansion, such as aluminium, a change in operating temperature causes dimensional changes to the filter. In order to maintain electrical characteristics of the filter when its dimensions change, it is known to provide a temperature compensating arrangement to compensate for the resulting resonant frequency shift.
It is also known to construct a cavity filter from a material having a low coefficient of thermal expansion such as Invar, a combination of nickel and iron. Filters made from this material provide very stable characteristics over a broad temperature range. However, it is not always practical to use filters made of invar because of the relatively heavy weight. Moreover, the thermal conductivity of invar is relatively poor which is a disadvantage in high power applications.
In order to avoid some of the drawbacks of filters made entirely of Invar, it is known to provide a filter arrangement in which the filter""s cavities comprise side walls of Invar and end walls of aluminium. Because of the difference between the coefficient of thermal expansion of the side walls and that of the end walls in this known type of arrangement, temperature compensation must be provided. One known method of providing temperature compensation to these filter arrangements is to provide the end walls with projections that extend into the filter""s cavities to reduce the volume change of a cavity that would otherwise occur due to expansion or contraction of the side walls with changes in temperature.
It is an object of the present invention to provide a relatively simple temperature compensating structure for a microwave cavity filter comprising at least one cavity having a side wall of a material having a low coefficient of thermal expansion and two end walls of a material having a relatively high coefficient of thermal expansion.
According to the invention there is provided a microwave filter comprising at least one cavity defined by a cylindrical side wall and two planar end walls having two opposite major surfaces whose perimeters are respectively attached by attachment means to an outwardly extending flange means at each end of said side wall, said side wall being made from a metallic material having a low coefficient of thermal expansion and said two end walls being made from a metallic material having a relatively high coefficient of thermal expansion, wherein a first continuous annular groove of a predetermined depth and a predetermined minimum width is provided in one major surface of each end wall proximate its perimeter, and a second continuous annular groove of predetermined depth and a predetermined minimum width is provided in the other major surface of each end wall proximate said perimeter, the diameter of said first annular groove being greater than the diameter of said second annular groove whereby a solid intermediate zone of predetermined width lies between the first and second annular grooves, and wherein a plurality of open ended slots of predetermined minimum width are provided in each said planar end wall at its perimeter, said slots extending from proximate said first annular groove to the outer boundary of said planar end wall.